Underground drilling, such as gas, oil, or geothermal drilling, generally involves drilling a bore through a formation deep in the earth. Such bores are formed by connecting a drill bit to long sections of pipe, referred to as a “drill pipe,” so as to form an assembly commonly referred to as a “drill string.” The drill string extends from the surface, to the bottom of the bore.
The drill bit is rotated so that the drill bit advances into the earth, thereby forming the bore. In a drilling technique commonly referred to as rotary drilling, the drill bit is rotated by rotating the drill string at the surface. In other words, the torque required to rotate the drill bit is generated above-ground, and is transferred to the drill bit by way of the drill string.
Drilling mud is a high pressure fluid that is pumped from the surface, through an internal passage in the drill string, and out through the drill bit. The drilling mud lubricates the drill bit, and flushes cuttings from the path of the drill bit. The drilling mud then flows to the surface through an annular passage formed between the drill string and the surface of the bore.
The drill bit can also be rotated by a mud motor driven by the flow of drilling fluid. The mud motor is usually mounted in the drill string, proximate the drill bit. The drill bit can be rotated by the mud motor alone, or by rotating the drill string while operating the mud motor.
So called “smart” drilling systems include sensors located down hole in the drill string. The information provided by these sensors permits the drill string operator to monitor relevant properties of the geological formations through which the drill bit penetrates. Based on an analysis of these properties, the drilling operator can decide to guide the drill string in a particular direction. In other words, rather than following a predetermined trajectory, the trajectory of the drill string can be adjusted in response to the properties of the underground formations encountered during the drilling operation. The technique is referred to as “geosteering.”
Various techniques have been developed for performing both straight hole and directional (steered) drilling without a need to reconfigure the bottom hole assembly of the drill string, i.e., the equipment located at or near the down-hole end of the drill string. For example, so called steerable systems use a mud motor with a bent housing. In such systems, the drill string is operated in a sliding mode in which the drill string is not rotated. Rather, the drill bit is rotated exclusively by the mud motor. The bent housing or subassembly steers the drill bit in the desired direction as the drill string slides through the bore, thereby effectuating directional drilling In prior art steerable system, the tool face angle of the bent housing could be adjusted from the surface by rotating the drill string, which reorients a scribe line whose position relative to the bend is known thereby allowing the operator to determine the amount of change in the tool face angle of the bent housing. However, unless the bottom hole assembly is pulled out of the bore hole and the bent housing removed when straight ahead drilling is desired, the use of the bent housing results in drilling an oversize bore hole when it is desired to return to straight drilling by rotating the drill string. Moreover the severity of the dog leg—that is, the degree of change in the angle of drilling—cannot be adjusted from the surface and requires pulling the bent housing out of the bore hole.
Other approaches to directional drilling, such as those described in U.S. Pat. Nos. 6,321,857 and 7,013,994, use cam surfaces or eccentric stabilizers, together with an eccentric mass, to induce an offset in the drill bit that alters the direction of drilling. Although the direction of drilling can be altered from the surface by rotating a mandrel that normally drives the drill bit to instead rotate the cam surface or eccentric stabilizers, the degree of the change in the drilling direction provided by the offset cannot be altered in such systems without pulling the drill string from the bore hole and changing the components. Nor can straight ahead drilling be accomplished without pulling the drill string from the bore hold and removing the directional drilling apparatus.
Other directional drilling systems, such as those disclosed in U.S. Pat. No. 7,762,356 (Turner et al), allow the direction of drilling, as well as the degree of change in the drilling direction, to be altered based on commands from the surface, without removing the drill string, by controlling the extension of members radially outward from the drill string that contact the side of the bore hole wall, for example once per revolution, so as to deflect the drill bit. However, such systems are complex and require precise control of the mechanism for extending the members that contact the bore hole wall.
Consequently, a need exists for an improved drilling system in which the change in the direction of drilling, including the degree of change in the drilling direction, can be effected from the surface, without removal of the drill string from the bore hole.